In discrete and integrated analog/mixed-signal circuits, signal sources can provide currents for which the difference carries certain sensor information. Specifically, in optical sensor systems where a light intensity difference must be measured, separate photo diodes with different spectral sensitivity or geometrical orientation provide light-intensity-proportional currents and can therefore be interpreted as a current source in this sense. The information of interest lies within the difference of these current signals.
U.S. Pat. No. 6,330,464 B1 and U.S. Pat. No. 7,289,836 B2 relate to an optical-based sensor for detecting the presence or amount of analyte using both indicator and reference channels. The sensor has a sensor body with an embedded source of radiation. Radiation emitted by the source interacts with the indicator membrane's molecules proximate the surface of the body. At least one optical characteristic of these indicator molecules varies with the analyte concentration. Radiation emitted or reflected by these indicator molecules enters and is internally reflected in the sensor body. Photosensitive elements within the sensor body generate both the indicator channel and reference channel signals to provide an accurate indication of concentration of the analyte. The difference between the two signals is utilized after their digitization.
Usually small photo currents must be processed. These small currents need to be amplified, which is typically done by a low-noise transimpedance amplifier (TIA). The TIA can provide output signals (e.g.) up to a finite level which is the saturation limit or the saturation voltage. Signals that go beyond this saturation limit will be clipped and thus distorted which prevents full-scale signal processing and therefore must be circumvented.
CA 2480608 relates to an elevated front-end amplifier offering low-noise performance while providing a wide dynamic range that is employed for amplifying the weak photo current received from a photo detector.
EP 0579751 B1 relates to a wideband TIA utilizing a differential amplifier circuit structure in which the differential pair is bridged by a signal detector that is the photo detector when the TIA is implemented within an optical receiver. In order to bias the signal detector, the differential pair is operated asymmetrically with respect to the DC voltage, but the circuit maintains a symmetric AC response to the signal detector current input. The circuit is designed to operate at the unity gain frequency. The signal detector is placed between the source (or emitter) electrodes of the transistors which helps to reduce the impact of gate (or base) capacitance on circuit response speed. Combined, these factors maximize the bandwidth capabilities of the circuit. The circuit is responsive to a current input to produce two voltage outputs equal in magnitude but opposite in phase.
“A CMOS Tunable Transimpedance Amplifier,” Hwang et al., IEEE Microwave and Wireless Component Letters, Vol. 16, No. 12Dec. 2006, relates to TIA that incorporates a mechanism for gain and bandwidth tuning. The TIA can be adjusted to achieve optimum performance with the lowest bit-error rate for high-speed applications.
“Low FPN High Gain Capacitive Transimpedance Amplifier for Low Noise CMOS Image Sensors”, Boyd Fowler, Janusz Balicki, Dana How, and Michael Godfrey, Pixel Devices Intl. Inc. relates to a low fixed pattern noise capacitive transimpedance amplifier (CTIA) for active pixel CMOS image sensors with high switchable gain and low read noise.
In order to interpret the current signals, an analog-to-digital converted and thus a digitized version of the analog signal must be provided. As in U.S. Pat. No. 6,330,464 B1 and U.S. Pat. No. 7,289,836 B2, two separate digital measurements values are obtained and their digital difference is processed further. The device utilized for the digitization is typically an analog-to-digital converter (ADC), which is always limited by a maximum resolution and processable dynamic range. In systems as described in U.S. Pat. No. 6,330,464 B1 and U.S. Pat. No. 7,289,836 B2, using (e.g.) RFID-based ISO-protocol-compliant parts that are powered via energy harvesting and thus do not come with an independent power supply, the overall duration of communication combined with the need for power to encode and modulate, etc., require that a host-slave interaction being as infrequent as possible. This is also required for communication security: the less often and shorter, the better. Additionally, RFID systems can be very sensitive to RF distortions, which might prevent communication or even a complete host-slave interaction. For these reasons, the transmission of one measurement result will always be preferred to transmitting two or more results.
Moreover, there are techniques based on correlated double-sampling. Correlated double sampling makes use of a subsequent sample, in time or function, of a current across a capacitor used to integrate different currents from the same source for use in compensating for offsets and low frequency noise effects; e.g., compensating for the dark current component of a pixel-photodiode in the overall desired light detection signal.
“A new correlated double sampling (CDS) technique for low voltage design environments in advanced CMOS Technology,” Chen Xu, ShenChao, Mansun Chan, ESSCIRC, September 2002, relates to a fixed-voltage-difference readout circuit implemented on a CMOS active pixel sensor.
“Correlated double sample design for CMOS image readout IC”, Gao Junet al., 7th International IEEE Conference on Solid-State and Integrated Circuits Technology (2004) relates to a two-amplifier state topology used for implementing a respective compensation method using a correlated double-sampling approach.
Because the A-D conversion is limited in dynamic range and is therefore also limited in the digital measurement results, the effective dynamic range and digital resolution, respectively, for the difference itself is less than for the individual signal current. The smaller the current signal's difference is, the less effective digital resolution is for the difference based on individually A-D converted separate current signals. Often a factor of 2 or less is present, for example, between a signal and reference current, which leads to a reduction of resolution of the signal of 1 bit or more.
Therefore, the problem arises of how to obtain an amplified, digitized difference between two separate current signals; whereas the full ADC resolution can be utilized to digitize the difference itself. If a TIA might be used as the processing element to resolving this problem, then it must be taken into consideration that the saturation limit of the TIA should not be reached throughout the whole processing time.
Moreover, the RF-transmission and especially the double A-D conversion typically consume more power than a single A-D conversion and respective measurement results transmission.
An additional problem lies in the limited absolute signal range and TIA gain in combination with the required gain to realize potential full-scale amplification. If the TIA gain is limited and a higher gain for the signal difference is required, single “saw-tooth-like” integration would not yield sufficient amplification. In this case, two subsequent single integration ramps with polar slope would lead to saturation and, hence, clipping of the individual current signal (current). Hence, the potential difference output would not be correct.